In a chromatograph apparatus, a set of data which represents a chromatogram with the horizontal axis indicating the time and the vertical axis indicating the signal intensity (e.g. output voltage) can be obtained by an analysis of a sample (such data are hereinafter called the “chromatogram data”). In a data processing system for a chromatograph, a peak which appears on such a chromatogram is detected and a substance corresponding to the peak is identified from the peak position (retention time) with reference to a previously set identification table. Furthermore, the concentration and/or quantity of the substance is calculated from the height or area of the peak.
Such a data processing system normally has some restrictions of the level of the signal that can be processed, due to hardware limitations on the signal-processing circuits including an A/D converter. For an input of a signal whose level is above the upper limit or below the lower limit, the system cannot perform correct calculations.
Besides such a limitation concerning the signal processing, there is another problem that a detection result obtained with a detector for a chromatograph apparatus varies with the signal level. For example, in a device used as a detector for a liquid chromatograph (such as an ultraviolet-visible spectrophotometer or photodiode array detector), the non-linearity of the signal intensity normally becomes more noticeable as the component concentration in the sample increases (as shown in FIG. 10), which lowers the accuracy of quantitative determination. Meanwhile, the signal inevitably has various noises superposed on it. Accordingly, when performing an analysis, it is preferable to prepare the sample so that the concentrations of its components will be included in a predetermined range (dynamic range).